One object in .rgw.buckets.index causes systemic instability

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Dear all,

I have a cluster running hammer (0.94.5), with 5 nodes. The main usage is for S3-compatible object storage.
I am getting to a very troublesome problem at a ceph cluster. A single object in the .rgw.buckets.index is not responding to request and takes a very long time while recovering after an osd restart. During this time, the OSDs where this object is mapped got heavily loaded, with high cpu as well as memory usage. At the same time, the directory /var/lib/ceph/osd/ceph-XX/current/omap gets a large number of entries ( > 10000), that won't decrease.

Very frequently, I get >100 blocked requests for this object, and the main OSD that stores it ends up accepting no other requests. Very frequently the OSD ends up crashing due to filestore timeout, and getting it up again is very troublesome - it usually has to run alone in the node for a long time, until the object gets recovered, somehow.

At the OSD logs, there are several entries like these:
 -7051> 2015-11-03 10:46:08.339283 7f776974f700 10 log_client  logged 2015-11-03 10:46:02.942023 osd.63 10.17.0.9:6857/2002 41 : cluster [WRN] slow re
quest 120.003081 seconds old, received at 2015-11-03 10:43:56.472825: osd_repop(osd.53.236531:7 34.7 8a7482ff/.dir.default.198764998.1/head//34 v 2369
84'22) currently commit_sent


2015-11-03 10:28:32.405265 7f0035982700  0 log_channel(cluster) log [WRN] : 97 slow requests, 1 included below; oldest blocked for > 2046.502848 secs
2015-11-03 10:28:32.405269 7f0035982700  0 log_channel(cluster) log [WRN] : slow request 1920.676998 seconds old, received at 2015-11-03 09:56:31.7282
24: osd_op(client.210508702.0:14696798 .dir.default.198764998.1 [call rgw.bucket_prepare_op] 15.8a7482ff ondisk+write+known_if_redirected e236956) cur
rently waiting for blocked object

Is there any way to go deeper into this problem, or to rebuild the .rgw index without loosing data? I currently have 30 TB of data in the cluster - most of it concentrated in a handful of buckets - that I can't loose.

Regards.
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